Various crude or refined hydrocarbons (e.g., methane, ethane, propane, etc.) can be pyrolized or cracked to synthesize hydrogen and to produce higher-order carbon substances (e.g., graphene and fullerenes). However, some of the processes used to produce these higher-order carbon substances require the use of catalysts, such as metal catalysts. Additionally, some processes result in the presence of impurities within the higher-order carbon substances. Furthermore, some processes require the formation of a “seed” or “core” around which the higher-order carbon substances are formed.
Different allotropes of carbon can be generated by cracking hydrocarbons utilizing thermal processes. One example of a process for generating lower-order carbon substances (e.g., carbon black) is the solar thermolysis of methane (both with and without a catalyst) to produce hydrogen and carbon black. An example of a process for generating higher-order carbon substances is the catalytic decomposition of methane in a quartz tubular reactor to produce hydrogen and highly graphitic carbon nanotubes, microfibers, microballs, and carbon onions.
Some examples of higher-order carbon allotropes are shown in FIG. 1. FIG. 1A shows a schematic of graphite, where carbon forms multiple layers of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex. Graphene is a single layer of graphite. FIG. 1B shows a schematic of a carbon nanotube, where carbon atoms form a hexagonal lattice that is curved into a cylinder. Carbon nanotubes can also be referred to as cylindrical fullerenes. FIG. 1C shows a schematic of a C60 buckminsterfullerene, where a single layer of a hexagonal lattice of carbon atoms forms a sphere. Other spherical fullerenes exist that contain single layers of hexagonal lattices of carbon atoms, and can contain 60 atoms, 70 atoms, or more than 70 atoms. FIG. 1D shows a schematic of a carbon nano-onion from U.S. Pat. No. 6,599,492, which contains multiple concentric layers of spherical fullerenes.